[Jake G]

Forgot to add the FEMA link: https://www.fema.gov/how-order-technical-administrative-support-data

– Jake G.

[Jake G]

Hi Charles,

In most cases, you need to order the FEMA backup data from the FEMA Engineering Library in order to get the FEMA model (cost is in the $400 range). They recently putting some recent models online, but that won’t apply to most situations yet. Also note that you can order the data and pay the money, but they find nothing in their archives–happens all the time.

Good luck,
Jake G.

[Jake G]

The HEC-RAS User’s Manual (Chapter 4) provides a nice step-by-step explanation for creating a 1-D steady flow model. HEC-RAS also comes with a good Applications Guide along with example projects you can install. Of course, once you’re ready to move on to 2-D modeling, you can follow the 2-D Modeling User’s Manual.

Good luck,
Jake G.

[Jake G]

Some doglegs are fine, but you don’t want an excessive number of doglegs and typically no 90-degree angles in your cross sections. Unless you’re just modeling low flows, think about overall flow direction during a large flood event rather than being focused on localized contour changes that won’t affect the overall flow. Along a bridge roadway, the bounding cross sections should typically be based on the overall flow direction during a large event rather than localized changes along the bridge/roadway.

Here’s a screenshot from an HEC publication (TD-39) that shows a good example of overall cross section layout.

[Jake G]

Hi Chris,

The culvert output table shows that the 500-year flow has Inlet Control at the culvert, while the rest of the profiles have Outlet Control. As a result, the 500-year profile experiences a supercritical flow regime through the culvert, which results in the crossing profiles. I don’t believe there is anything intrinsically wrong with it hydraulically.

Please see page 6-9 of the HEC-RAS Hydraulic Reference Manual for more details about RAS and inlet vs. outlet control.

Good luck!

– Jake Gusman

p.s. While I didn’t look at the model in detail, the cross sections, especially adjacent to the culvert seems to have too many right angles and/or “doglegs” in them. You may want to reassess those.

[Jake G]

Hi Craig,

Take a look at Keith M’s first response to a similar question at this link:

http://hec-ras-help.1091112.n5.nabble.com/Modeling-culvert-2D-Area-td3891.html

This same solution has worked for me in the past.

Good luck,
Jake G.

[Jake G]

Hi Mike,

In a similar situation, I modeled it as two separate culverts in series. The upstream section of the culvert was modeled with an entrance loss, but no exit loss, while the downstream culvert section was modeled with no entrance loss, but with an exit loss.

The cross section in the middle of the culverts can be modeled with a lid.

– Jake G.

[Jake G]

Hi Ange,

You can do this in HEC-RAS, but the cross section points should be station-elevation rather than station-depth. One approach would be to use an arbitrary elevation for the crest, say 1000, and then subtract the depths from that value (you can also just subtract them from 0 if you don’t mind negative elevations).

When you run the model, you can use the “Set Changes in WS and EG…” option in the Flow Data Editor to fix the water surface elevation at the crest elevation for each of the cross sections behind the dam.

In the output table, you can use the Define Table option to select the Volume variable, which computes a cumulative volume. Or you can use the computed flow area at each cross section along with the reach lengths to compute the volume.

Hope this helps.

– Jake G

[Jake G]

Hi Silvia,

I’m sure you have fixed your model by now, but if not, a screenshot would help with understanding what issue you’re having.

If the issue is that the water surface profile is below the ground profile on the plot, that’s usually because the area bounded by channel bank stations doesn’t include the lowest portion of the cross section.

Thanks,
Jake G

[Jake G]

Hi Susan,

The FHWA (2012) publication Hydraulic Design of Safe Bridges provides a great, up-to-date refresher course on 1D and 2D modeling.

https://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=1&id=153

It’s not specific to any particular model, but lays the foundation really well.

Good luck,
Jake G.

[Jake G]

Hi Lauren,

You should be able to use the table variable “Min Ch El Sta”. Note that if there are multiple points at the same invert elevation, then I believe it takes the rightmost point for the station.

Good luck,
Jake G.

[Jake G]

Hi Antoine,

It sounds like you want to use the Fixed Sediment Elevations under Tools in the Geometric Data Editor. You can apply sediment to one or multiple cross sections, either at individual elevations or at a slope across the reach.

– Jake G.

[Jake G]

Jervin,

Assuming the study reach is experiencing subcritical flow, it is typical for a constriction to cause a decrease in computed water surface elevation and an increase in velocity at the location of the constriction. However, you will often see an increase in WSEL further upstream of the constriction.

– Jake

[Jake G]

igorce,

To build on Keith’s excellent points, here are a couple of additional observations:

(a) Freeboard (safety height) is usually computed on the upstream side of a bridge because that is where debris typically accumulates.

(b) Supercritical flow would only be expected in limited situations, such as a concrete channel or steep bedrock channel. Supercritical or mixed flow should not be used for most natural channels. See, for example, Jarrett (1984):

http://ascelibrary.org/doi/abs/10.1061/(ASCE)0733-9429(1984)110:11(1519)

– Jake G.

[Jake G]

Colin,

That sounds like a reasonable approach to editing the model, although you want to make sure the ends of the clipped cross sections still contain the peak flow. Also, in some cases it’s better to reduce the number of cross sections within a sharp bend.

Typically if you are removing points in the Cross Section Data editor and accepting the changes to the cut line, the bank stations should remain in place. They tend to get messed up if you’re using Remove Points from the Edit menu instead.

– Jake G.

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